For decades the dairy powder industry has been faced with the challenge of producing low thermophile and low spore dairy powder products. The presence of some level of thermophiles in dairy powder is inevitable because of their presence in the incoming raw milk. However, due to the exponential growth behavior of the thermophiles this minimum level can quickly grow to unacceptable levels over the course of a production cycle because the processing equipment provides an ideal growth environment. Concentration of the thermophiles and spore counts as the milk product solids are concentrated by evaporation and spray drying further multiplies the thermophile and spore counts by a factor of 7.5 or more. Consequently, there is a significant need for a system and method capable of consistently producing low thermophile and low spore dairy powder throughout a production cycle without having to repeatedly shut down the entire system for cleaning.
Thermophiles in milk powders are undesired because they produce heat resistant spore which are difficult to destroy, produce constituents which may detract product functionality, and spores and vegetative bacteria may cause spoilage in application. For these reasons and more milk powder customers do not want high thermophile and spore counts in their milk powders.
Biofilm is a primary cause of high thermophile counts in dairy powders. Biofilm is formed in processing equipment when a microbial bacteria cell attaches to the interior surface on which it remains in contact with the product and then the cell begins to grow and divides forming more bacteria cells. The number of bacteria cells increases rapidly and the bacteria cells begin to form micro colonies. This growth stage is referred to as the maturation period. Within about 9-12 hours the micro colonies begin to disperse cells, which can be carried further downstream in the process where they become attached and trigger the growth of additional micro colonies. In the first 9-12 hours, cell detachment from the biofilm is not a major issue. However, as shown in FIG. 1A, once detachment from the biofilm starts, the thermophile/spore count in the milk that is in contact with the biofilm increases rapidly. The thermophile/spore count is not the only thing that increases rapidly, the fouling rate of the processing equipment also increases. FIG. 1B shows magnitudinal development of the fouling rate and the spore count in an evaporator over 36 hours of continuous operation. As shown in FIG. 1A the spore count begins growing rapidly beyond approximately 9 to 12 hours along with the fouling rate. As a result, cleaning of the evaporator becomes increasingly more difficult the longer the operating run, therefore requiring more chemicals and longer cleaning cycles.
High temperature short time (HTST) pasteurization of milk will kill a portion of the living thermophilic bacteria that contaminates the milk leaving the heat exchangers of the preheating system, and higher temperature heat treatment steps will kill more of the bacteria and reduce the living thermophile population in the milk. However, a more significant long-term concern remains the “dormant” thermophile spores that are also released into the milk by the biofilm. The “dormant” thermophile spores are not killed by pasteurization and other heat treatment steps and these “dormant” thermophile spores will grow in the finished milk powder once sufficient moisture is re-introduced, even months after spray drying. An additional concern is that the concentration of the milk product in the evaporator simultaneously concentrates the population of these spores, which only compounds the problem of final spore contamination levels in the finished milk powder.
In response to this problem the dairy industry has used a variety of different techniques to try and limit the spore contamination in the finished milk powder. One of the traditional techniques includes, for example, installing a dual feed preheating system for the evaporator system and switching the feed flow to the second “clean” feed preheat system after approximately 10 hours of operation when the biofilm formed in the first preheat system has reached maturity. The first preheat system is then “isolated” for the remainder of the 20-hour evaporator processing day and may be independently cleaned in place (CIP) or with both feed preheat systems being CIP cleaned with the evaporator at the end of the 20 hour day to remove the matured biofilms and any other product fouling or residues.
Although this approach has been proven to reduce the thermophile and spore contamination of milk powders produced when running the evaporator system continuously for a 20-hour run, the 20-hours of continuous low spore operation remains a challenge that requires extraordinary discipline for everyday repeatability. Furthermore, switching the dual feed preheating system can cause solid concentration upsets in the evaporator and cause the HTST system to divert, which can lead to undesired disruption of the spray dryer operation. Also, it does not address the problem of thermophile biofilms which form and continue to grow in the downstream evaporator equipment, which adds to the contamination of the final milk powder with both living thermophilic bacteria and spores. This downstream biofilm issue can be partially mitigated by running the front end of the evaporator at elevated temperatures at which biofilm growth rates are reduced, but this does not entirely address the problem.
In addition, this approach of operating the evaporator system for 20 hours and then CIP cleaning for 4 to 5 hours requires the evaporator system and spray dryer to be offline up to 20% of every day, during which time the spray dryer is not producing milk powder. Some facilities may have two or more evaporator systems feeding one or more spray dryers to achieve nearly perpetual spray dryer operation, but this requires significant capital investment in equipment, and a significant period of the time a portion of the equipment will be sitting idle. Capital investment in equipment that sits idle for extended periods of time is an economically infeasible option for many producers.
Furthermore, frequent starting and stopping of the spray dryer is undesired because startup and shutdown can be complex and decreases efficiency. Instead, the most desired and efficient manner of operation for a spray dryer is continuous operation for periods exceeding, for example, several days or even weeks. Continuous operation of the spray dryer has become increasingly more coveted in recent years as the demand for milk powders has surged worldwide, particularly in Asia. As a result of the surge in demand, milk powder producers have been trying to boost production capacity any way they can.
Some producers have attempted to eliminate the 4-5 hour shutdown of the spray dryer during the evaporator cleaning by storing a volume of milk concentrate in a buffer tank between the evaporator and the spray dryer. However, milk concentrate at 50% TS (i.e., typical spray dryer feed concentration) cannot be held in a buffer tank for very long without age thickening of the concentrate if the milk is stored hot, or lactose crystallization and gelling if the milk concentrate is stored cold. This problem becomes exacerbated when the milk product has been heat treated to a medium or high heat specification. As a result, this has not been found to be a practical solution.
Another technique tested for addressing the thermophile and spore growth problem was frequent shutdowns for CIP cleaning of the entire evaporator system, but this still creates the issue of frequent starting and stopping of equipment which is particularly impractical for the spray dryer and which causes a significant reduction in net production time for the entire processing line.
In another effort to maximize production, some milk powder producers partially concede to the thermophile and spore growth by choosing to continue a production run despite the likelihood of high thermophile and spore counts. In these situations the producer may separate the milk powder produced earlier in the production run from the milk powder produced later and then separately package and offer it for sale as different quality.
In consideration of the aforementioned circumstances, and the shortcomings of all the prior attempted solutions by those in the industry, the present disclosure provides a new evaporator and heat treatment system and method for producing low thermophile and low spore milk powders capable of continuous spray dryer operation. The method for producing low thermophile and low spore milk powders of the present disclosure also applies successfully to more traditional 20 hour spray dryer milk powder production cycles where only one (1) finisher evaporator and one (1) spray dryer feed system are available to supply 50% TS concentrate feed to the spray dryer.